JP2007157639A - Metal separator for fuel cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a metal separator for fuel cell having a contact resistance lower than that of conventional one and a manufacturing method capable of obtaining this separator without using ammonia gas. <P>SOLUTION: This is a metal separator for fuel cell in which Ti<SB>2</SB>N exists on the uppermost surface of an electrolyte electrode assembly. To be concrete, it is advisable that it has a base layer made of a metal material and a surface layer laminated on the electrolyte electrode assembly side of the base layer, and Ti<SB>2</SB>N exists on the uppermost surface of the surface layer. On the other hand, the separator can be obtained by going through a process in which Ti existing on the surface of the base layer of metal material is nitrided in a nitrogen atmosphere using nitrogen for the inlet gas. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell metal separator and a method for producing the same.

  A fuel cell is a power generator for converting chemical energy directly into electrical energy. As a fuel cell, for example, a polymer electrolyte fuel cell (PEFC) using a solid polymer electrolyte as an electrolyte is widely known.

  In general, a fuel cell has a structure in which a fuel electrode is joined to one surface of an electrolyte and an oxidant electrode is joined to the other surface to form an electrolyte electrode assembly, and a plurality of these are laminated via separators.

  Of the constituent members of the fuel cell, the separator is often exposed to a corrosive environment such as an acidic atmosphere or an oxidizing atmosphere. Moreover, a separator contacts with the electrode of an electrolyte electrode assembly and collects electricity. Therefore, the separator is mainly required to have characteristics such as having good corrosion resistance, conductivity, and low contact resistance.

  Conventionally, for example, in PEFC, carbon has been used as a separator material that satisfies the above required characteristics. However, the carbon separator has a problem that the processability is poor and the gas flow path has to be formed by cutting, so that the cost tends to be high.

  Therefore, there has been a demand for replacing the separator material with a metal material that has good processability and is cheaper than carbon.

  Therefore, in recent years, for example, a metal separator has been proposed in which stainless steel having good corrosion resistance is used as a base material layer, and a conductive layer such as gold is laminated on the surface of the base material layer. In addition to stainless steel, there is a movement to use titanium or the like for the base material layer.

  Under such a background, for example, Patent Document 1 includes a base material layer made of stainless steel or titanium and a protective layer covering the base material layer, and the surface of the protective layer is occupied by TiN. A metal separator is disclosed in which the N content decreases as it goes inside.

Moreover, as a manufacturing method of this metal separator, a titanium layer is formed on the surface of a base material layer made of stainless steel, and a nitrogen mixed gas having a composition of N ₂ : H ₃ : CO ₂ = 1: 1: 0.07 is used. A method for forming a protective layer having the above structure by nitriding a titanium layer in a nitrogen atmosphere is disclosed.

Japanese Unexamined Patent Publication No. 2000-353531 (Example 2, Fig. 10 etc.)

  However, the metal separator and its manufacturing method described in Patent Document 1 still have room for improvement in the following respects.

  That is, according to previous studies by the present inventors, the metal separator described in Patent Document 1 has a problem of relatively high contact resistance because the surface of the protective layer in contact with the electrode is TiN. It has also been found that this problem becomes significant when the protective layer is exposed to a corrosive environment.

On the other hand, in Patent Document 1, it is presumed that ammonia gas is used to manufacture the metal separator. This is because, firstly, it is common practice to use ammonia gas to produce TiN by nitriding titanium, and secondly, the composition of the nitrogen gas mixture to be used contains an H ₃ component. For reasons such as being.

  By the way, ammonia gas is known to be toxic. Therefore, there are many restrictions on the use of ammonia gas due to environmental problems. Therefore, it can be said that it is desirable not to use ammonia gas as much as possible in manufacturing the metal separator.

  Therefore, the problem to be solved by the present invention is to provide a metal separator for a fuel cell having a contact resistance lower than that of the prior art. Another object is to provide a production method capable of obtaining the fuel cell metal separator without using ammonia gas.

In order to solve the above problems, the fuel cell separator according to the present invention is characterized in that Ti ₂ N exists on the outermost surface on the electrolyte electrode assembly side.

In this case, the fuel cell separator includes a base material layer made of a metal material and a surface layer laminated on the electrolyte electrode assembly side of the base material layer, and Ti ₂ N is present on the outermost surface of the surface layer. It is good to make it.

The area ratio of Ti ₂ N is preferably 50% or more.

  On the other hand, the method for producing a fuel cell metal separator according to the present invention includes a step of nitriding Ti present on the surface of a base material layer made of a metal material in a nitrogen atmosphere using nitrogen as an introduction gas. This is the gist.

  At this time, the nitriding temperature is preferably in the range of 700 ° C. to 1050 ° C.

In the metal separator for a fuel cell according to the present invention, Ti ₂ N is present on the outermost surface on the electrolyte electrode assembly side. Therefore, the contact resistance between the electrode of the electrolyte electrode assembly and the outermost surface of the separator is smaller than in the past. Moreover, even when exposed to a corrosive environment, fluctuations in contact resistance can be suppressed.

Such an effect is obtained because Ti ₂ N, which is superior in conductivity to TiN and has a corrosion resistance equivalent to or higher than that of TiN, is actively present on the outermost surface of the separator, and the electrolyte electrode assembly This is because they are in direct contact with the electrodes.

  In addition, the fuel cell metal separator is advantageous in terms of cost compared to a conventional fuel cell metal separator using gold or the like.

At this time, when the area ratio of Ti ₂ N is 50% or more, there is an advantage that the above effect can be easily obtained.

  On the other hand, the method for producing a fuel cell metal separator according to the present invention includes a step of nitriding Ti present on the surface of a base material layer made of a metal material in a nitrogen atmosphere using nitrogen as an introduction gas. .

Therefore, a surface layer in which Ti ₂ N is present on the outermost surface is formed on the surface of the base material layer made of a metal material. Therefore, according to the manufacturing method described above, a metal separator for a fuel cell having a lower contact resistance with the electrode than before can be obtained without using ammonia gas as the introduction gas.

At this time, when the nitriding temperature of Ti is in the range of 700 ° C. to 1050 ° C., there is an advantage that Ti ₂ N is likely to be dominant on the outermost surface of the surface layer.

  Hereinafter, the metal separator for a fuel cell according to the present embodiment and the manufacturing method thereof will be described in detail. Hereinafter, the metal separator for a fuel cell according to the present embodiment may be referred to as “the present separator”, and the method for producing the metal separator for a fuel cell according to the present embodiment may be referred to as “the present production method”.

1. This separator is made of metal, and conductive Ti ₂ N is present on the outermost surface on the electrolyte electrode assembly side. That is, this separator is characterized in that Ti ₂ N is positively extracted from the outermost surface on the electrolyte electrode assembly side.

  The reason for adopting such a configuration is mainly as follows. In general, a fuel cell has a structure in which both surfaces of an electrolyte electrode assembly in which electrodes are bonded to both surfaces of an electrolyte are sandwiched by a pair of separators, and a plurality of these are stacked.

In such a laminated structure, the separator is usually used in a state where one surface thereof is in contact with the electrode surface of the electrolyte electrode assembly. For this reason, in this separator, Ti ₂ N is present at least on the outermost surface that is in direct contact with the electrolyte electrode assembly, so that the contact resistance is reduced while having corrosion resistance.

Here, the Ti ₂ N preferably occupies most of the outermost surface of the separator.

However, it is often difficult in manufacturing to make all of the outermost surface of the separator completely Ti ₂ N. Therefore, as long as the object of the present invention is not impaired, the outermost surface of the separator includes titanium nitride other than Ti ₂ N, such as TiN and TiN _0.3 , Ti, inevitable impurities, and the like. It does not matter if it exists. That is, the Ti ₂ N only needs to exist predominantly on the outermost surface of the separator.

In this separator, in the case of PEFC, for example, in the case of PEFC, specifically, the area ratio of Ti ₂ N in the surface in contact with the gas diffusion layer is preferably 50% or more, more preferably 60% or more, more preferably 70% or more. This is because if the area ratio of Ti ₂ N is within the above range, the effect of the present invention can be easily ensured.

In the present separator, as described above, the Ti ₂ N may be present on the outermost surface that is in direct contact with the electrode surface, but is present not only on the outermost surface but also on a certain distance from the outermost surface to the inside. You may do it.

In this case, the presence state of Ti ₂ N inside the outermost surface is not particularly limited. This is mainly based on the purpose of this separator.

  Specifically, for example, it may be distributed so that the existence ratio decreases from the outermost surface to the inside, or it is distributed at substantially the same existence ratio over a certain distance from the outermost surface to the inside. Also good.

The presence of Ti ₂ N on the outermost surface can be confirmed by analyzing the outermost surface of the separator by X-ray diffraction or the like.

  This separator is basically composed mainly of metal. The metal here includes not only a single metal but also an alloy. The metal to be used can be appropriately selected in consideration of corrosion resistance and the like.

  Specific examples of the metal that mainly constitutes the separator include, for example, corrosion resistant metals such as stainless steel, titanium, and titanium alloys, and more specifically, metals that passivate in the environment in the separator and exhibit corrosion resistance. can do. These may be used alone or in combination of two or more.

  As a specific configuration of the separator, for example, a configuration including a base layer made of the metal material and a surface layer laminated on the electrolyte electrode assembly side of the base layer can be exemplified.

In this case, at least Ti ₂ N is present on the outermost surface of the surface layer. Further, the surface layer may be inclined so that the proportion of Ti ₂ N existing from the outermost surface toward the base material layer is reduced, or substantially the same from the outermost surface to the base material layer surface. Ti ₂ N may be distributed in proportion.

  At this time, the thickness of the surface layer is not particularly limited as long as the object of the present invention can be achieved. Specifically, for example, examples of preferable upper limit values include 100 μm, 50 μm, and 10 μm. On the other hand, examples of preferable lower limit values that can be combined with these preferable upper limit values include 0.1 μm, 1 μm, and 5 μm.

  The thickness of the base material layer can be appropriately selected in consideration of gas separation properties, structural strength, corrosion resistance, weight reduction, and the like.

  Specifically, for example, 0.8 mm, 0.5 mm, 0.3 mm and the like can be exemplified as preferable upper limit values. On the other hand, examples of preferable lower limit values that can be combined with these preferable upper limit values include 0.05 mm, 0.1 mm, and 0.2 mm.

  The separator is formed with a gas flow path for supplying fuel gas and oxidant gas. Such a gas flow path can be formed by, for example, plastic working such as press molding. As specific gas flow channel configurations, for example, known configurations may be employed as appropriate, and detailed description thereof is omitted.

  The separator can be applied to a fuel cell that operates at a temperature at which a metal member can be used. Examples of fuel cells include polymer electrolyte fuel cells (including PEFC and direct methanol fuel cells (DMFC)), solid oxide fuel cells (SOFC), and proton conductive glass such as phosphate glass as an electrolyte. The fuel cell etc. which were used for can be illustrated.

  The separator can maintain a relatively low contact resistance even when exposed to a corrosive environment. Therefore, for example, a strongly acidic group is used as an electrolyte group, and can be suitably used for PEFC and the like that are likely to be in a strongly acidic environment.

2. This Manufacturing Method This manufacturing method basically includes a step of nitriding Ti existing on the surface of the base material layer in a nitrogen atmosphere (hereinafter, also referred to as “this step”).

In this step, nitrogen is used as an introduction gas to create the nitrogen atmosphere. Even when ammonia is used as the introduced gas, a nitrogen atmosphere can be created by N ₂ generated by the decomposition of ammonia, but the nitrogen atmosphere in this step is distinguished from the atmosphere created in this way.

That is, in this step, an introduction gas substantially filled with N ₂ is used, and ammonia gas is not included in the introduction gas. Note that “substantially” means that it is difficult to use 100% pure nitrogen gas, and therefore it may contain inevitable impurities. Further, other gas than ammonia gas may be included as long as it does not adversely affect the formation of Ti ₂ N.

In this step, since such a nitrogen atmosphere is used, Ti ₂ N is formed when Ti present on the surface of the base material layer is nitrided.

  Here, in this step, when the metal material constituting the base material layer is made of Ti or a Ti alloy, a Ti component is present on the surface of the base material. Therefore, in this case, this Ti component can be used as Ti existing on the surface of the base material layer.

  On the other hand, when the base material layer is made of a metal material other than Ti or Ti alloy, for example, stainless steel, Ti needs to be present on the surface of the base material layer by some method.

  Specifically, as a technique for causing Ti or Ti alloy to be present on the surface of the base material layer made of a metal material other than Ti or Ti alloy, for example, cladding, electroplating, thermal spraying, physical vapor deposition (PVD: vacuum) Examples thereof include a vapor deposition method, a sputtering method, an ion plating method, and a chemical vapor deposition method (CVD: thermal CVD, plasma CVD, etc.). These methods may be used alone or in combination of two or more. Preferably, cladding, electroplating, or the like that does not require evacuation is advantageous in terms of cost.

  However, when the base material layer is made of Ti or a Ti alloy, it can be said that it is more advantageous in terms of cost because the step itself of allowing Ti or Ti alloy to exist on the surface of the base material can be omitted.

  In addition, before nitriding Ti, an oxide film formed on the surface of the base material layer may be removed as necessary. Specific examples of the removal method include plasma etching, chemical etching, and mechanical polishing.

Further, in this step, the nitriding temperature of Ti may be appropriately selected as the optimum temperature at which Ti on the surface of the base material layer is nitrided and Ti ₂ N is synthesized.

  Specific examples of the preferable upper limit of the nitriding temperature include 1050 ° C., 880 ° C., and 800 ° C. On the other hand, specific examples of preferable lower limit values that can be combined with these preferable upper limit values include 700 ° C. and 750 ° C., for example.

  The nitriding time may be appropriately selected in consideration of the nitriding temperature and the like. For example, when emphasis is placed on improving productivity, the surface layer can be selected in a relatively short time at a high temperature at which nitriding reaction is likely to occur.

  Further, as described above, a gas flow path or the like is usually formed in the separator. The step of forming the gas flow path may be performed before this step or after this step.

  To explain using a specific example, Ti undergoes a phase transition from α-type to β-type at a temperature of about 880 ° C. Therefore, when importance is attached to dimensional accuracy and the like, since the phase transition of Ti does not occur when the nitriding temperature is 880 ° C. or lower, the step of forming the gas flow path and this step may be performed in any order.

  On the other hand, when the nitriding temperature is 880 ° C. or higher, the heat treatment exceeding the phase transition temperature is performed when this step is performed after the gas flow path is formed. Therefore, the dimensional accuracy of the obtained separator may be reduced.

  Therefore, at this time, a separator with high dimensional accuracy can be obtained by performing this step after forming the gas flow path.

  This process described above can be performed, for example, in an apparatus capable of performing PVD, an apparatus capable of performing CVD, or a continuous heating furnace.

  Among these, this process is preferably performed in a continuous heating furnace. This is because it is a relatively simple device and advantageous in terms of cost.

Hereinafter, the present invention will be described more specifically with reference to examples.
Example 1
First, a pure Ti plate having a thickness of 0.3 mm was press-formed to form a gas flow path. Next, this press-molded body was exposed in a nitrogen atmosphere using nitrogen as the introduced gas, and Ti on the surface on the electrolyte electrode assembly side was gas-nitrided.

  At this time, the supply amount of nitrogen was 0.5 L / min, the heating temperature of the pure Ti plate was 800 ° C., and the heating time was 2 hours.

  Thus, a Ti separator according to Example 1 was produced.

(Example 2)
A pure Ti plate having a thickness of 0.5 mm was exposed in a nitrogen atmosphere using nitrogen as the introduced gas, and Ti on the surface on the electrolyte electrode assembly side was gas-nitrided.

  At this time, the supply amount of nitrogen was 0.5 L / min, the heating temperature of the pure Ti plate was 1000 ° C., and the heating time was 0.5 hours. Thereafter, the obtained plate material was press-molded to form a gas flow path.

  Thus, a Ti separator according to Example 2 was produced.

(Comparative Example 1)
A commercially available pure Ti plate having a thickness of 0.5 mm, the surface of which was coated with TiN (AIP method: arc ion plating method) was prepared. Subsequently, this was press-molded to form a gas flow path. Thus, a Ti separator according to Comparative Example 1 was produced.

(Comparative Example 2)
A commercially available pure Ti plate having a thickness of 0.5 mm, the surface of which was coated with TiN (HCD method: follow cathode deposition method) was prepared. Subsequently, this was press-molded to form a gas flow path. Thus, a Ti separator according to Comparative Example 2 was produced.

  Next, in the above Examples and Comparative Examples, each sample (outer diameter: 34 mm) having titanium nitride formed on the surface was prepared. And using these samples, the X-ray diffraction pattern measurement, the electrolytic corrosion test, and the contact resistance value before and after the electrolytic corrosion test were measured.

(X-ray diffraction pattern measurement)
X-ray diffraction patterns were measured by applying X-rays to the surfaces of the samples according to Examples 1 and 2 and Comparative Examples 1 and 2. The obtained X-ray diffraction pattern is shown in FIG.

  According to FIG. 1, it can be seen that the outermost surfaces of the samples according to Comparative Example 1 and Comparative Example 2 are covered with TiN.

On the other hand, it can be seen that Ti ₂ N is present on the outermost surface of each sample according to Example 1 and Example 2. However, TiN and the like are present in addition to Ti ₂ N on the outermost surface of each sample. However, it can be seen that the peak intensity of Ti ₂ N is stronger than these peak intensity.

Therefore, according to this test result, it can be seen that the main component of the outermost surface of each sample of Example 1 and Example 2 is Ti ₂ N, and Ti ₂ N exists predominantly.

(Electrolytic corrosion test)
Next, an electrolytic corrosion test was performed on each sample according to Example 1 and Comparative Examples 1 and 2. At this time, the electrolytic solution, Cl dilute sulfuric acid ^- the 10 ppm, F ^- was 5ppm added, it was used to prepare the ph to 4.

  An electrolytic cell having an inner diameter of 34 mm and a height of 30 mm was used, the measurement temperature was 80 ° C., the voltage (vs. Pt) was 0.26 V, and the test time was 1 day.

  2 to 4 show the electrolytic corrosion test results of the samples according to Example 1 and Comparative Examples 1 and 2. FIG. 2 to 4, it can be seen that the corrosion current suddenly drops in the initial stage and is constant at a certain level.

  According to this test result, it was confirmed that Example 1 and Comparative Examples 1 and 2 have corrosion resistance.

  Next, for each sample according to Example 1 and Comparative Examples 1 and 2, the contact resistance value with the carbon paper before and after the electrolytic corrosion test was measured by the following procedure.

  FIG. 5 is a diagram schematically showing a method for measuring the contact resistance value. That is, a low-current DC power source with each sample 14 and carbon paper 16 (simulating an electrode of an electrolyte electrode assembly as a contact partner) sandwiched between the gold-plated copper plate 10 and the gold-plated copper plate 12 A steady current of 1 A was applied by 18. At this time, a load of 1.47 MPa in air pressure was applied to each of the gold-plated copper plates 10 and 12 by a load cell (not shown).

  As a result, a current of 1 A flows through the contact portion (contact area 2 cm × 2 cm) between each sample 14 and the carbon paper 16. Therefore, the electrical resistance value was measured from the potential difference between each sample 14 and the carbon paper 16 60 seconds after the load was applied, and this was used as the contact resistance value.

  In FIG. 6, the contact resistance value with the carbon paper before and after the electrolytic corrosion test of Example 1 and Comparative Examples 1 and 2 is shown.

  According to FIG. 6, it can be seen that Example 1 has a lower contact resistance value before the electrolytic corrosion test than Comparative Examples 1 and 2. Further, it can be seen that Example 1 has a lower contact resistance value after the electrolytic corrosion test and less fluctuation in the contact resistance value before and after the test than Comparative Examples 1 and 2.

  From the above test results, it was confirmed that the separators according to the examples were excellent in corrosion resistance, had low contact resistance, and could suppress fluctuations in contact resistance even when corrosion occurred. Moreover, it has also been confirmed that it is not necessary to use ammonia gas to obtain the separator according to the example that exhibits such effects.

  The fuel cell metal separator and the manufacturing method thereof according to the present invention have been described above. However, the present invention can be variously modified without departing from the spirit of the present invention.

3 is an X-ray diffraction pattern of each sample according to Examples 1 and 2 and Comparative Examples 1 and 2. FIG. It is the figure which showed the electrolytic corrosion test result of the sample which concerns on Example 1. FIG. It is the figure which showed the electrolytic corrosion test result of the sample concerning the comparative example 1. It is the figure which showed the electrolytic corrosion test result of the sample concerning the comparative example 2. It is the figure which showed typically the measuring method of a contact resistance value. It is the figure which showed the contact resistance value with the carbon paper before and behind an electrolytic corrosion test about the sample which concerns on Example 1 and Comparative Examples 1 and 2. FIG.

Claims (5)

A fuel cell metal separator, wherein Ti ₂ N is present on the outermost surface on the electrolyte electrode assembly side. A base material layer made of a metal material;
A surface layer laminated on the electrolyte electrode assembly side of the base material layer,
The fuel cell metal separator according to claim 1, wherein Ti ₂ N is present on the outermost surface of the surface layer. The metal separator for a fuel cell according to claim 1 or 2, wherein the area ratio of Ti ₂ N is 50% or more.   A method for producing a metal separator for a fuel cell, comprising a step of nitriding Ti existing on the surface of a base material layer made of a metal material in a nitrogen atmosphere using nitrogen as an introduction gas.   The method for producing a metal separator for a fuel cell according to claim 4, wherein the nitriding temperature is in a range of 700 ° C to 1050 ° C.

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